The usage of power switching circuits has become ubiquitous in the electronics industry. A few examples of applications are switching power supplies, DC-DC voltage converters and DC-AC voltage converters.
During the operation of such power switching circuits, a frequently encountered situation is when power switching devices such as MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors) are turned off against unclamped or partially clamped inductive loads. During the turn off, a high rate of load current change, abbreviated as di/dt, is often impressed upon these inductive loads due to the gate drive of the power MOSFET device. This normally results in an associated voltage overshoot and ringing as the leakage inductive energy stored in the unclamped circuit inductances resonates with parasitic circuit capacitances before dying down. Excessive ringing can cause power losses and excessive peak voltages from the voltage overshoot can drive the power MOSFET device into avalanche breakdown with the risk of permanent device failure. Additionally, the voltage overshoot and ringing can also result in high levels of conducted and/or radiated EMI/RFI (electromagnetic interference/radio frequency interference) causing undesirable interference with the operation of other sensitive electronic systems nearby.
A number of studies have been conducted of the voltage overshoot and ringing phenomenon so as to minimize its adverse effects. Subsequent solutions include adding snubbers across the MOSFETs, slowing down the turn off speed by reducing the gate turn off current, etc. Such solutions typically either require many additional components and/or are inefficient. Therefore, there exists a need to cost effectively reduce such associated voltage overshoot and ringing as caused by the high rate of load current change di/dt. The following is a list of references relevant to the understanding of the present invention:    1. F Merienne, J Roudet, J. L. Schanen, “Switching disturbance due to source inductance for a power MOSFET: analysis and solutions”, IEEE Power Electronics Specialists Conference, PESC 1996 Record, Vol 2, pp 1743-1747.    2. G Nobauer, D Ahlers and J Ruiz-Sevillano, “A method to determine parasitic inductances in Buck Converter topologies” Infineon Application Note, June 2004.    3. Qun Zhao, Goran Stojcic, “Characterization of Cdv/dt induced power loss in Synchronous Buck DC-DC converters”, IEEE Applied Power Electronics Conference, APEC 2004, Vol 1, pp 292-297.    4. Bo Yang, Jason Zhang “Effect and Utilization of Common Source Inductance in Synchronous Rectification”, IEEE Applied Power Electronics Conference, APEC 2005, Vol 3, pp 1407-1411.    5. W Teulings, J. L. Schanen, J Roudet, “MOSFET switching behavior under influence of PCB stray inductance”, IEEE Industry Applications Conference, 1996. Vol 3, pp 1449-1453.